1. Field of the Invention
The present invention relates to a photocurable liquid developer used in an image forming apparatus utilizing an electrophotographic method such as electrophotography, electrostatic recording or electrostatic printing, and a photocurable liquid developer used in an inkjet recording apparatus (hereinafter, these photocurable liquid developers will be referred to also as “liquid developers”); a method for producing a liquid developer; and a developing device and an image forming apparatus which use a liquid developer.
2. Description of the Related Art
The electrophotographic method is a method for obtaining printed matter by uniformly charging the surface of an image bearing member such as a photoconductor (a charging step), exposing the surface of the image bearing member so as to form a latent electrostatic image on the surface (an exposing step), developing the formed latent electrostatic image with a developer including colored resin particles (a developing step), transferring the developer image to a recording medium such as paper or OHP sheet (a transfer step), and fixing the transferred developer image to the recording medium (a fixing step). In this case, usable developers are broadly classified into dry developers in which colored resin particles formed of materials including binder resins and colorants such as pigments are used in a dried state, and liquid developers obtained by dispersing colored resin particles in electrically insulating liquids.
Nowadays, there is an increasing need for image colorization with respect to image forming apparatuses utilizing the electrophotographic method, such as copiers, facsimiles and printers. Color printing involves, for example, printing a high-resolution, high-quality image such as a photograph, for which reproduction of a vivid color tone is required; accordingly, a developer capable of meeting such requirements is demanded. Further, on the market, there is a new demand for increase in processing speeds (the term “processing speeds” refers to development processing speed, transfer processing speed and fixation processing speed), which does not cause decrease in image quality.
Dry developers, for which developers in a solid state are used, are popular among developers at present because of their advantageousness in terms of handleability. To obtain a high-resolution, high-quality image, a developer is required to have chargeability which is appropriate for attachment of the developer in an amount corresponding to the charge density of a latent electrostatic image formed on an image bearing member. However, the dry developers present problems with the environmental stability of their chargeability in terms of prevention of image degradation caused by environmental changes such as temperature change and humidity change; moreover, the dry developers easily cause aggregation of colored resin particles, for example while stored, and thus present problems with the uniformity, etc. of the colored resin particles when the colored resin particles are dispersed. Also regarding these properties, the above-mentioned problems, caused by the dry developers being in powder form, become more noticeable when the colored resin particles are made relatively small in particle diameter in an attempt to achieve high resolution.
Liquid developers, meanwhile, include electrically insulating liquids as carrier liquids; thus, they do not easily cause the problem of aggregation of colored resin particles therein while stored, in comparison with the dry developers, and therefore use of fine toners is possible. As a result, the liquid developers are superior to the dry developers in terms of reproducibility of images containing thin lines, tone reproducibility and color reproducibility, and can be superiorly employed in image forming methods involving high speed processing. Development of high-image-quality, high-speed digital printing apparatuses that utilize electrophotographic techniques using liquid developers, which takes advantage of the foregoing superior features, is becoming more and more active. Under these circumstances, development of liquid developers with better properties is demanded.
Examples of known conventional liquid developers include a developer obtained by dispersing colored resin particles in an electrically insulating liquid such as silicone oil. However, such a liquid developer hinders bonding of colored resin particles at the time of image fixation owing to the presence of the electrically insulating liquid, and thus the developer may not be favorably fixed to a recording medium. Accordingly, it is necessary to remove the electrically insulating liquid using a plurality of removal rollers before image fixation, which causes an image forming apparatus with the liquid developer to be complex and makes it difficult to meet the demand for high-speed processing.
As a countermeasure against the foregoing problems, there has been proposed a method of curing an electrically insulating liquid. For example, Japanese Patent Application Laid-Open (JP-A) No. 2007-525717 proposes a method of fixing colored resin particles by cross-linking a silicone oil serving as an electrically insulating liquid. However, this proposed method utilizes an oxidation reaction, a hydrosilylation reaction, a condensation reaction, etc., so that there is a limitation to the reaction rate and it is therefore difficult to meet the demand for high-speed processing.
Further, there has been proposed a method of curing an electrically insulating liquid by photopolymerization. A photocurable liquid developer in this case includes reactive functional group-containing monomer(s)/oligomer(s) as an electrically insulating liquid, and a photopolymerization initiator is added and dissolved in the electrically insulating liquid. By irradiating this photocurable liquid developer with light, e.g., ultraviolet rays, the developer cures due to a polymerization reaction, thereby making it possible to adapt to high-speed processing. Examples of such a photocurable liquid developer include the one described in Japanese Patent (JP-B) No. 3442406. In JP-B No. 3442406, a curable liquid vehicle with a specific viscosity range and a specific resistance value range is disclosed as a curable electrically insulating liquid, and the curable liquid vehicle is exemplified by a (meth)acrylic-modified silicone. More specifically, there is a description saying that a silicone portion contains an aliphatic/aromatic siloxane chain or ring with a specific dimethylsiloxane unit.
Meanwhile, JP-B No. 4150118 discloses inclusion of a reactive silicone compound in a curable electrically insulating liquid, and examples of the reactive silicone compound include silicone compounds having, in their molecules, functional groups such as isocyanate group, Si—H group, vinyl group, amino group, hydroxyl group, epoxy group, methacryloxy group, etc.
However, the (meth)acrylic-modified silicones described in JP-B Nos. 3442406 and 4150118 are highly photocurable but present a serious problem in terms of safety. Vinyl monomers such as styrene monomers and (meth)acrylic monomers generally have strong reactivity, are potentially dangerous, may irritate the skin or eyes, may become hypersensitivity-inducing substances (sensitizing substances), possibly induce allergy and have strong, unpleasant smells; as a result of all this, the uses of the vinyl monomers are restricted or the vinyl monomers are prohibited from being used. Moreover, the vinyl group-containing silicone compound exemplifying the reactive silicone compound, mentioned in JP-B No. 4150118, does not sufficiently cure by ultraviolet irradiation and is therefore problematic in terms of fixability.